Score tomatoes with an X on the bottom and place in hot water for 1-2 minutes until skins soften. Peel the tomatoes and cut into cubes. Peel, core, and dice the apples.
Heat some oil in a pan over medium heat. Add finely chopped onion and cook for 2-3 minutes until softened. Add tomato cubes, apple cubes, and broth cube. Reduce heat to low and simmer for 10 minutes.
While the sauce simmers, coat fish fillets with lemon juice and season with salt. Add the fish and sour cream to the pan and simmer for another 10 minutes until the fish is cooked through.
Garnish with parsley, chives, or cress and serve with boiled potatoes and salad.